1. Field of the Invention
This invention relates to a voltage-controlled oscillator employed, for example, in a multichannel microwave transmission apparatus.
2. Description of the Related Arts
As a voltage-controlled oscillator, a silicon bipolar transistor generating less low-frequency-noise has been employed in a lower-frequency band, for example, below 12 GHz, and a gallium-arsenide field effect transistor (referred to hereinafter as a GaAs FET) has been preferably employed in a higher-frequency band due to its high maximum frequency of oscillation though its low-frequency noise characteristics is not preferable. The low-frequency noise of the transistor vitally affects a phase-noise characteristics of the oscillated frequency. Accordingly, the phase-noise characteristics of the microwave oscillator employing a GaAs FET is generally worse than that of silicon transistor.
A typical circuit diagram of a prior art microwave voltage-controlled oscillator employing a GaAs FET and its noise characteristics are shown in FIG. 1 and FIG. 2, respectively. Drain electrode D of a GaAs FET 14 is grounded, its gate electrode G is connected to a transmission line 13 whose opposite end is grounded via a DC (direct current) stopping capacitor C.sub.1 and a terminating resistor R.sub.1. An oscillation frequency signal f.sub.out is output from its source electrode S via a DC stopping capacitor C.sub.2. Transmission line 13 is electromagnetically coupled with a dielectric resonator 12 having a resonant frequency f.sub.o. Dielectric resonator 12 is further coupled with an open stub 11 whose one end is open and another end is connected to a variable-capacitance diode D.sub.1 to which a reverse bias is applied via a choke coil CH.sub.1. choke coils CH.sub.2 and CH.sub.3 supply a bias voltage to gate electrode G, and a source voltage to source electrode S, respectively. Open stub 11 and variable-capacitance diode D.sub.1 form a sub-resonator circuit and modulates, i.e. modifies, the resonant frequency f.sub.o of dielectric resonator 12 by the capacitance change according to the reverse voltage applied to the diode D.sub.1. Thus, this oscillator operates as a voltage-controlled oscillator providing a frequency change of, for example, .+-.30 MHz for 12 GHz centre frequency.
GaAs FET 14 having worse low-frequency noise characteristics compared with that of a silicon transistor generates phase noise components denoted with A on the sidebands, namely .+-.1 MHz, of the output frequency, 12 GHz, as shown as frequency spectrum in FIG. 2. Therefore, it has been seriously requested to provide a microwave voltage-controlled oscillator excellent in its phase-noise characteristics employing a transistor which is not good in a low-frequency noise characteristic.